Various techniques have been devised to test subterranean fluids. In the hydrocarbon recovery industry, coiled tubing units are commonly used to transmit sample fluids from a particular zone in the well to the surface for testing of the fluid. This procedure may be performed either in open (uncased) boreholes, or in cased boreholes which includes a sliding sleeve which selectively opens a port to allow communication between the formation and both the interior of the casing and the lower end of the coiled tubing. To prevent contamination between subterranean zones, inflatable packers, grout or cement may be used to seal the casing above and below the sliding sleeve. The coil tubing technique is, however, expensive and time-consuming. In order to reduce expense, some oil recovery operators use wireline test tools which can be quickly and inexpensively lowered into a wellbore to test fluid samples without retrieving the samples to the surface. This procedure is, however, frequently considered unreliable.
U.S. Pat. No. 4,222,438 discloses a fluid sampling procedure to determine downhole conditions in a fluid-producing subterranean reservoir. A downhole test tool is suspended from a wireline, and engages a shoulder at the lower end of a tubing string to position the test tool at a desired depth. U.S. Pat. No. 4,535,843 discloses a wireline tool for sampling borehole fluids and transmitting preliminary sample results to the surface. Based on these preliminary results, the operator may determine if samples should be collected in the tool and retrieved to the surface. The downhole tool includes a hydraulic pump and motor for inflating a double packer and either drawing sample fluid into a container chamber within the tool, or rejecting the sample fluid into the borehole. A control system to reduce the number of solenoid valves in a downhole sample tool is disclosed in U.S. Pat. No. 4,573,532.
While various downhole fluid sampling techniques have the common purpose of testing fluids, unique problems are presented when sampling for particular properties and when sampling under particular conditions. When testing for the presence and/or level of contamination of water in subterranean formations adjacent nuclear plants, nuclear test sites, or landfills, the high reliability required of the testing procedure may necessitate that the sampled fluid be brought to the surface, either for testing or for verification of any downhole testing. Also, the testing procedure must prevent any contamination of the subterranean formation. Relatively small diameter test wells have been drilled surrounding such sites for the purpose of determining whether any, and hopefully ensuring that no, contamination is migrating out of the test site with subterranean fluids, such as water. To conduct meaningful tests, each subterranean zone which might possibly be in fluid communication with the subterranean mass known to be contaminated must be checked. Since each test well may have multiple test zones which should be tested several times a typical year for a period of 20 years, and since there are numerous potentially contaminated sites located both within and outside the United States, subterranean tests must be reliably performed at a reasonable cost.
A significant difficulty with devising a subterranean migration test as outlined above is a requirement that any fluids withdrawn from the formation not be allowed to be returned to either a withdrawn zone or any other subterranean zone. Fluid may be contaminated, and its injection into another zone would mean that the testing procedure itself has caused increased contamination. Also, if any fluid is reinjected in a zone, it is likely that this reinjected fluid will be withdrawn and re-tested, so that over time the same fluid is repeatedly tested and a true indication of zone contamination at a particular well site is not obtained. Also, the very act of reinjecting any fluid in any zone may alter the normal flow of subterranean fluids, thereby invalidating the test.
One procedure which has been suggested for migration testing is to drill a test well to the depth of each zone to be tested, and then case the test well down to the test depth. While this procedure minimizes the chances of contamination between zones, it requires a separate well to test each zone at a particular well site, which is not cost feasible. Another alternative is to drill a single well at each test site with a sliding sleeve in the casing at the depth of each zone to be tested. To test the zone at a particular depth, a tool may be used to mechanically open the sliding sleeve at this depth. If the well is evacuated and a plug positioned in the bottom of the well, water in the zone will flow into the well when the sliding sleeve is opened. After the sliding sleeve is shifted closed, test fluid within the well may be withdrawn to the surface by injecting nitrogen gas through coiled tubing inserted into the well, thereby lifting the fluid to the surface in the annulus between the casing and the coiled tubing. Since any test fluid in any zone obtained at any time might contaminate subsequent tests, the entire well must be pumped dry above the bottom test plug, and care taken to ensure that the well was completely evacuated. This procedure is also costly and time-consuming, and requires the storage of a significant quantity of water until tests can verify that the fluid is not contaminated. The entire quantity of fluid withdrawn during each test must then be handled by proper treatment or disposal techniques.
The disadvantages of the prior art are overcome by the present invention. Improved methods and apparatus are hereinafter disclosed for inexpensively and reliably conducting subterranean fluid tests. While the techniques of the present invention may be used for testing various fluids for various purposes, the invention is particularly well suited for determining whether there is migration from a particular site with the subterranean fluids, such as water.